Neurons form precise and complex neural circuits to generate perception and behavior. A major goal of neuroscience is to map the large ensemble of neural circuits in the brain and understand how they account for normal brain functions and neurological disorders. The small size and optical transparency of larval zebrafish make it an ideal system to investigate neural circuits in an intact vertebrate organism. However, current techniques for mapping neural circuitry in zebrafish are limited in speed and scale, preventing the full realization of zebrafish's potential for neural circuit studies. To address this gap in technology, we propose to develop a virus-based toolkit that would enable rapid and systematic neural circuit mapping in zebrafish. In preliminary studies, we established that vesicular stomatitis virus (VSV) can be used as a tool to map neural circuits. It efficiently infects zebrafih neurons, spreads rapidly across synapses, and expresses fluorescent reporters for circuit mapping. VSV labeling could also be combined with other imaging methods to probe neuronal function. To extend these studies, we propose three specific aims: Aim 1. We will establish virus-based tools to map neural circuits on a whole-brain scale, in vivo. We will investigate the characteristics of VSV and distribute reagents and neural circuit mapping results to the community. Aim 2. We will develop tools to elucidate connectivity patterns of specific neuronal types. These tools will be able to (1) target initial infection to defined cell types and (2) identfy neurons that are directly connected to them. Aim 3. We will create tools that combine neural circuit mapping with functional analyses. These tools will help bridge the gap between neural circuit structure and function. In summary, this proposal addresses a critical need for zebrafish neural circuit tracing tools and provides a novel approach to combine anatomical and functional analyses. We will utilize these tools to map the circuitry underlying visual function, both as a tet platform and as a resource for studying vision. These tools could be applied throughout the nervous system and facilitate neural circuit studies in the wider community, e.g. neural development, physiology, behavior, regeneration, as well as disease models.